Magnetic-type EAS systems have become commonplace in the last decade or so, being primarily used in protecting books in libraries, bookstores, etc., where such systems offer certain advantages over EAS systems operating on other principles, e.g., "RF" or "microwave" based systems. It is thus well known that such magnetic-type EAS systems typically comprise a transmitting means for producing, within an interrogation zone, a magnetic field which alternates at a predetermined frequency, markers adapted to be affixed to articles to be protected, each such marker containing a low coercive force, high permeability ferromagnetic material which responds to the interrogation field by producing harmonics of the predetermined frequency, and a detecting means for producing an appropriate alarm signal when selected harmonics are detected. Such systems are, for example, described in U.S. Pat. No. 3,665,449 (Elder et al.) and subsequent related patents, and have been marketed by Minnesota Mining and Manufacturing Company (3M) as TATTLE TAPE brand EAS systems.
The markers used in such systems have typically comprised elongated strips of polycrystalline, low coercive force, high permeability material, such as permalloy, Supermalloy, etc. (see U.S. Pat. No. 3,790,945, Fearon, and subsequent patents). It is also known to use amorphous materials having similar magnetic properties. See RE 32,427 and 32,428. Elongated strips have been used in such markers to alleviate demagnetization effects which otherwise inhibit the production of readily distinguished, very high order harmonics. While it is also suggested in the '449 patent that other shapes, such as thin, flat discs having a ratio of major dimension to thickness of at least 6,000, may similarly have a low demagnetization factor and, hence, be a useful shape for an EAS marker, such shapes have never become commercially viable.
However, the desirability of a disc, square or rectangular-shaped marker has not escaped notice. For example, it has been recognized that a response similar to that obtained from an elongated shape could be produced in a square piece of high permeability, low coercive form magnetic material by configuring the square piece into a plurality of flux collector portions and restricted cross-sectional area switching sections. Thus, while the demagnetization factor within the switching section was unfavorable, such that an inadequate response would be expected, the addition of the flux collectors caused sufficient flux to be concentrated within the switching section and overcame the otherwise unfavorable shape. See U.S. Pat. No. 4,710,754 (Montean).
Still others have sought to provide markers utilizing thin-films. Thus, for example, Fearon, U.S. Pat. No. 4,539,558 (Col. 16, lines 2-14), has proposed that an elongated marker may be formed of a strip of alternating sputtered layers of ferromagnetic materials. In that construction, each layer is separated by an evaporated coating of, for example, aluminum oxide. Fearon still emphasizes the necessity of an elongated shape and the subsequent need for appropriate orientation in an interrogation field. In a later patent (U.S. Pat. No. 4,682,154), Fearon also suggests that markers responsive in the gigahertz frequency range may include multiple micro-thin sputtered layers of ferromagnetic material, with each layer being separated by an insulating layer, such as gadolinium oxide or holmium oxide. Each of the individual ferromagnetic layers is required to be so thin as to no longer exhibit ferromagnetic behavior at room temperature. The composite layers, sandwiched between alternate layers of insulating material, is thus said to exhibit excellent ferromagnetic characteristics at the super high frequency range. Thus, for example, the individual sputtered layers are therein proposed to be about three atom layers thick.
More relevant to the present invention, it has also been proposed to overcome the demagnetization problem, which otherwise necessitates elongated marker construction, by providing a thin film of an amorphous, zero magnetostriction, ferromagnetic material. Such a thin-film, typically in the range of 1-5 um thick, is proposed to be deposited by sputtering onto an acceptable synthetic polymeric substrate, such as polyimide. See, for example, EP Application No. 295,028 (Pettigrew). A preferred construction as there set forth, having a thickness of 1 um and dimensions in the plane of the film of 3 cm by 2 cm, would have a ratio of major dimension to thickness of 20,000, thus exceeding the lower bound of 6,000 acknowledged in Elder (U.S. Pat. No. 3,665,449).